BBH merger rate density as a function of redshift for different BBH formation channels. The solid black lines show the merger rate density of the whole BBH population. The shaded area encompasses the difference between M_min = 10⁶ M_⊙ (upper boundary), and M_min = 10⁸ M_⊙ (lower boundary). The different panels show the results obtained assuming different CE α parameters. All the models assume the fiducial SFR–M_* relation P23 and the fundamental metallicity relation by Andrews & Martini (2013). The gray-shaded regions show the MRD inferred by the LVK collaboration (Abbott et al. 2023b).